1. Field of the Invention
This invention relates to an optical disc apparatus, a focus position control method and an optical disc that can suitably find applications in the field of optical disc apparatus for recording holograms on optical discs.
2. Description of the Related Art
An optical disc apparatus for irradiating a light beam onto an optical disc such as a Compact Disc (CD), a Digital Versatile Disc (DVD) or a Blu-ray disc (trademark, to be referred to as BD herein after) and reproducing information by reading light reflected from the optical disc have become highly popular.
Such optical disc apparatus can also record information on an optical disc by irradiating a light beam onto the optical disc, locally changing the reflectivity or the like of the optical disc.
It is known that the size of light spot formed on the optical disc is defined to be approximately equal to λ/NA (λ: wavelength of light beam, NA: numerical aperture) and the resolution is also proportional to that value. For example, Y. Kasami, Y. Kuroda, K. Seo, O. Kawakubo, S. Takagawa, M. Ono and M. Yamada, Jpn. J. Appl. Phys., 39, 756 (2000) describes in detail a BD that has a diameter of 120 mm and can record about 25 GB of data.
Meanwhile, optical discs are adapted to record information of various types such as various contents including music contents and image contents and various data to be used in computers. Particularly, in recent years, the quantity of information to be handled at a time has increased due to availability of high definition images and high sound quality music and a single optical disc is required to record a large number of sets of contents. Thus, there is an increasing demand for optical discs having a larger recording capacity.
As an attempt to meet the demand, techniques for increasing the recording capacity of a single optical disc by laying two or more than two recording layers on a single optical disc have been proposed (see, inter alia, I. Ichimura et al., Technical Digest of ISOM' 04, pp. 52, Oct. 11-15, 2005, Jeju Korea.
On the other hand, optical disc apparatus for using holograms on optical discs as recording technique have been proposed (see, inter alia, R. R. McLeod et al., “Microholographic Multilayer Optical Disk Data Storage”, Appl. Opt., Vol. 44, 2005, p.p. 3197.
For example, as shown in FIG. 1, a known optical disc apparatus 1 is designed to converge a light beam emitted from an optical head 7 into an optical disc 8 typically made of photopolymer whose refractive index changes according to the intensity of irradiated light and subsequently converge the light beam once again to the same focus position from the opposite direction by means of a reflection device 9 arranged on the rear surface side of the optical disc 8 (the lower side in FIG. 1).
More specifically, the optical disc apparatus 1 drives a laser 2 to emit a light beam that is a laser beam, modulates the light wave by means of an acousto-optical modulator 3 and collimates the light beam by means of a collimator lens 4. Subsequently, the light beam is transmitted through a polarization beam splitter 5 and turned from linearly polarized light into circularly polarized light by a quarter wavelength plate 6 before it is made to enter the optical head 7.
The optical head 7 is designed to record and reproduce information. It reflects the light beam by means of a mirror 7A, condenses it by means of an objective lens 7B and irradiates it onto the optical disc 8 that is being driven to rotate by a spindle motor (not shown).
The light beam is focused once to the inside of the optical disc 8 and reflected by the reflection device 9 arranged on the rear surface side of the optical disc 8 and then converged to the same focus from the rear surface side in the inside of the optical disc 8. Note that the reflection device 9 is formed by using a condenser lens 9A, a shutter 9B, a condenser lens 9C and a mirror 9D.
Thus, as a result, a standing wave is produced at the focus position of the light beam to form a recording mark RM that is a small hologram having a size of a light spot and a profile formed by bonding two circular cones together at the bottoms thereof. Then, the recording mark RM is recorded as information.
When the optical disc apparatus 1 records a plurality of such recording marks RM in the inside of the optical disc 8, it forms a mark recording layer by arranging the recording marks RM along concentric or spiral tracks, rotating the optical disc 8. Then, the optical disc apparatus 1 can lay a plurality of mark recording layers one on the other to record recording marks RM by adjusting the focus position of the light beam.
Thus, as a result, the optical disc 8 shows a multilayer structure having a plurality of mark recording layers in the inside. For example, in the case of the optical disc 8 shown in FIG. 2B, the distance between adjacent recording marks RM (mark pitch) p1 is 1.5 μm and the distance between two adjacent tracks (track pitch) p2 is 2 μm, while the distance between two adjacent layers p3 is 22.5 μm.
When the optical disc apparatus 1 reproduces information from the optical disc 8 where recording marks RM are recorded, it closes the shutter 9B of the reflection device 9 so that no light beam is irradiated from the rear surface side of the optical disc 8.
At this time, the optical disc apparatus 1 drives the optical head 7 to irradiate a light beam onto a recording mark RM in the optical disc 8 and has the reproduction light beam produced from the recording mark RM enter the optical head 7. The reproduction light beam is turned from circularly polarized light into linearly polarized light by means of the quarter wavelength plate and reflected by the polarization beam splitter 5. Furthermore, the reproduction light beam is condensed by a condenser lens 10 and irradiated onto a photodetector 12 by way of a pinhole 11.
Then, the optical disc apparatus 1 detects the quantity of light of the reproduction light beam by means of the photodetector 12 and reproduces information on the basis of the results of the detection.